Nitric oxide (NO) has diverse roles both in normal and pathological processes including the regulation of blood pressure, in neurotransmission, and in the macrophage defense systems (Snyder, S. H., et al., Scientific American, May 1992, 68). NO is synthesized by three isoforms of nitric oxide synthase (NOS), two of which, one in endothelial cells (eNOS) and one in neuronal cells (nNOS), are constitutive, and the one, in macrophage cells, which is inducible (iNOS). These enzymes are homodimeric proteins that catalyzed a five-electron oxidation of L-arginine, yielding NO and citrulline. The role of NO produced by each of the NOS isoforms is quite unique. Overstimulation or overproduction of individual NOS isoforms plays a role in several disorders including septic shock, arthritis, diabetes, ischemia-reperfusion injury, pain and various neurodegenerative diseases (Kerwin, J. F. Jr., et al., J. Med. Chem. 1995, 38, 4343). For example, the role of NO in cerebral ischemia can be protective or destructive depending on the stage of evolution of the ischemic process and on the cellular compartment producing NO (Dalkara, T., et al. Brain Pathology, 1994, 4, 49). While the NO produced by eNOS is likely beneficial by acting as a vasodilator to improve blood flow to the affected area (Huang, Z., et al. J. Cereb. Blood Flow Metab. 1996, 16, 981), NO produced by nNOS may contribute to the initial metabolic deterioration of the ischemic penumbra, resulting in larger infarcts (Hara, H., et al., J. Cereb. Blood Flow Metab. 1996, 16, 605). The metabolic derangement that occurs during ischemia and subsequent reperfusion results in the expression and release of several cytokines that activate iNOS in several cell types including some of the central nervous system. NO can be produced at cytotoxic levels by iNOS, and increased levels of iNOS contribute to progressive tissue damage in the penumbra, leading to larger infarcts (Parmentier, S., et al. Br. J. Pharmacol., 1999, 127, 546). Inhibition of i-NOS has been shown to ameliorate cerebral ischemic damage in rats (Am. J. Physiol., 268, R286 1995).
NO produced by I-NOS is also thought to play a role in diseases that involve systemic hypotension such as toxic shock and therapy with certain cytokines. It has been shown that cancer patients treated with cytokines such as interleukin 1 (IL-1), interleukin 2 (IL-2) or tumor necrosis factor (TNF) suffer cytokine-induced shock and hypotension due to NO produced from macrophages, i.e., inducible NOS (i-NOS) (Chemical & Engineering News, December 20, 33, 1993). i-NOS inhibitors can reverse this. Suppression of adjuvant induced arthritis by selective inhibition of i-NOS is reported in Eur. J. Pharmacol., 273, p. 15-24 (1995).
n-NOS inhibition has also been shown to be effective in antinociception, as evidenced by activity in the late phase of the formalin-induced hindpaw licking and acetic acid-induced abdominal constriction assays (Br. J. Pharmacol., 110, 219-224, 1993). Finally, opioid withdrawal in rodents has been reported to be reduced by n-NOS inhibition (see Neuropsychopharmacol., 13, 269-293,1995).
NOS inhibitors can be therapeutic in many disorders, but preservation of physiologically important nitric oxide synthase function requires the development of isoform-selective inhibitors.